In order to calculate this you just have to remember the rule for the least common multiple, if you want to calculate the least common facto you can multiply directly the two numbers in this case the least common multiple is 4*7=28, if you think there is a smaller common factor you can se the multiple of both numbers:
4-8-12-16-20-24-28
7-14-21-28
As you can see the multiplication between the two numbers is the least common factor and that would be the customer that gets both the pen and the pad of paper.
